A solar pond power plant comprises two major elements: a solar pond for collecting and storing solar radiation incident on the pond, and a power plant that utilizes a low-temperature turbogenerator for converting heat extracted from the pond into electricity.
One form of solar pond comprises an upper, convective wind-mixed layer exposed to solar radiation with a depth that varies from 10-50 cm, depending on weather conditions, and having salinity that varies from 3-5%; a non-convective intermediate halocline about 1-1.5 m deep whose salinity increases with depth to a maximum of about 25-30% for absorbing heat from solar radiation passing through the wind-mixed layer; and a lower heat storage layer about 3-5 m deep and of 25-30% uniform salinity. Solar radiation penetrating the wind-mixed layer and the halocline is absorbed in the heat storage layer. Radiation absorbed in the halocline establishes therein a temperature gradient that matches the salinity gradient, causing the halocline to act as an insulator for the heat storage layer and thereby preventing heat absorbed by the heat storage layer from being lost to the atmosphere by convection. The lowermost layer in the pond is a stratified thermocline which limits transfer of heat to the ground under the solar pond.
A suitable low-temperature turbogenerator for a solar pond power plant comprises a generator driven by a vapor turbine such as a low-pressure steam turbine, or an organic fluid turbine operating on a closed Rankine cycle. The power plant itself includes a boiler formed of a heat exchanger through which hot brine from the heat storage layer is pumped, the cooled brine being returned to the heat storage layer at a point isolated from the point at which the brine is drawn into the boiler. For an organic fluid turbine, the heat exchanger contains Freon or other similar fluid, which is vaporized by the hot brine in the heat exchanger, the vaporized working fluid being supplied to a turbine within which the vaporized working fluid expands for driving the generator. The working fluid exhausted from the turbine is then directed into a condenser where the working fluid is condensed and returned by a pump to the heat exchanger, thus completing the working fluid cycle.
In one arrangement, the condenser is cooled by water drawn from the wind-mixed layer, the warmed water produced by the condenser being returned to the surface of the pond where the heat absorbed in the condenser is dissipated into the atmosphere. Other arrangements for the condenser are also possible, of course, where cooling water other than the pond is available.
In southern California and at comparable latitudes, the solar input to a pond averages about 250 watts/m.sup.2 of pond area (on a 24-hour, yearly basis). From past experience, the heat input to the heat storage layer is about 40 watts/m.sup.2. Taking into account the various efficiencies of thermal to electrical conversion of an organic fluid Rankine-cycle turbogenerator, the net electrical output, in terms of heat, for producing energy at the same rate at which heat is absorbed in the heat storage layer is about 3 watts/m.sup.2. Thus, a pond one square kilometer in size would furnish sufficient heat to produce about 26 million KWh of electricity per year.
Based on actual experience in constructing and operating a 7,500 m.sup.2 solar pond power plant at EinBokek, Israel, pond construction costs are estimated to be about $12.00 per square meter, and equipment costs are estimated to be about $900.00 per installed kilowatt. For a 3 MW solar pond power plant using a one-square-kilometer pond, the total cost is estimated to be about $5,000.00 per installed kilowatt. This figure is about five times greater than the cost of an installed kilowatt in a conventional coal-fired plant, and about fifteen times greater than the cost of an installed kilowatt in a gas turbine plant. Even though cost economies in pond construction within the next five years should cut the installed kilowatt cost by 50%, investment in solar pond power plants is justifiable only on the basis of fuel savings.
By reason of the current high cost of fuel, the likelihood of even higher costs in future years, and the possibility of interruption in supply of this fuel, regional electric generating systems, which have installed capacity in the thousands of megawatts utilizing large-scale fossil and nuclear fueled power plants, as well as hydroelectric plants, are considering the integration of solar pond power plants into their systems. As is well known, regional electrical generating systems generally utilize their newest and most efficient plants (i.e., nuclear plants and hydroelectric plants, as well as coal plants) to supply the base power load of the system being served inasmuch as these plants operate for extended periods of time and have the lowest unit cost of production. Intermediate loads on the system, which are relatively large loads that are somewhat time-variable according to daily or seasonal demands, are conventionally furnished by older and less efficient plants that are brought on line as needed to supplement the output of the base load power plants. Peak power requirements in excess of the base and intermediate loads constitute a relatively small percentage of the total power output of a system, and operate for relatively short periods during a day. Consequently, peak power is conventionally supplied by relatively inefficient but highly reliable gas turbine power plants that can be brought on line or removed from service quickly in accordance with system load requirements.
Because the concept of solar pond power plants is so new, little experience is available on which to base decisions on integrating a solar pond power plant into a regional electrical generating system. Consequently, it is an object of the present invention to provide a new and improved solar pond power plant and method for operating the same as a part of an electrical generating system.